Communication systems can generally be divided into two broad classes: analog communication systems and digital communication systems. Both types of communication systems are known to comprise a plurality of communication units (e.g., in-car mobile and hand-held portable radios), base stations, and dispatch positions (e.g., consoles and console interface units). In an analog communication system, communications between users of the system typically comprise voice information, sourced by the communication units and/or the dispatch positions, represented throughout the system as analog signals. For example, when communicating with a particular communication unit, the audio supplied by the operator of the dispatch position is converted to an analog representation, which representation is sent to a base station and used to modulate a communication medium, such as a radio frequency (RF) channel. Upon reception of the modulated communication medium, the communication unit demodulates the communication medium to recover, and subsequently render audible, the analog representation of the audio information.
Alternately, digital communication systems often use a compressed digital representation in place of the analog representation described above. While digital representations of audio information offer many advantages over analog representations, they do present some disadvantages. Among these disadvantages is the difficulty in summing multiple audio sources together. For instance, in many analog communication systems, a communication unit may desire to concurrently monitor communications from both a dispatch position and other communication units. To accomplish this, a base station, acting as a repeater, electrically sums the multiple analog representations from the dispatch position and other communication units. The resulting summed analog representation is then transmitted to the communication unit, which renders the summed representation audible as previously described.
Such a summing technique, however, is not as easily implemented in digital communication systems utilizing compressed digital representations. Most compressed digital representations of audio information are not directly summable. That is, an audio source represented in a particular compressed digital format cannot be directly summed (e.g., through the use of digital addition) with another audio source using the same compressed digital format.
A solution to this summation problem is to decompress those signals intended for summation. Decompression of compressed digital signals converts them into a format allowing the direct summation of multiple signals. Thus, using the previous example of concurrent monitoring, the decompressed signals can be summed together and subsequently converted back into the compressed digital format for transmission to the monitoring communication unit. Once received by the communication unit, the compressed signal, comprising the summed audio information, is again decompressed an rendered audible.
A shortcoming, however, of this solution is that multiple compressions and decompressions of audio signals can significantly degrade their audio quality. Furthermore, the additional processing power required, as well as the added system throughput delay, make this solution prohibitive. Therefore, a method is needed which allows concurrent monitoring of multiple communications to be performed by a communication unit in a digital communication system utilizing a compressed digital format.